Fiber lasers, fiber optics for communication systems, and other systems for light delivery, such as in medical, industrial and remote sensing applications, often handle high levels of optical power, namely, up to several Watts in a single fiber or waveguide. When these high specific intensities or power per unit area are introduced into the systems, many thin film coatings, optical adhesives and even bulk materials, are exposed to light fluxes beyond their damage thresholds and are eventually damaged. Another issue of concern in such high-power systems is laser safety, where well-defined upper limits are established for powers emitted from fibers. These two difficulties call for a passive device that will switch off the power propagating in a fiber or waveguide, when the power exceeds the allowed intensity. Such a switching device should be placed either at the input of a sensitive optical device, or at the output of a high-power device such as a laser or an optical amplifier, or integrated within an optical device.
In the past, there have been attempts to realize an optical safety shutter, mainly for high-power laser radiation and high-power pulsed radiation; special efforts were devoted to optical sights and eye safety devices. The properties on which these prior art solutions were based included: (1) self-focusing or self-defocusing, due to a high electric-field-induced index change through the third order susceptibility term of the optical material, and (2) reducing the optical quality of a gas or a solid transparent insert positioned at the cross-over spot of a telescope, by creating a light-absorbing plasma in the cross-over point. These are described in U.S. Pat. No. 3,433,555 and U.S. Pat. No. 5,017,769. U.S. Pat. No. 3,433,555 describes a plasma that is created in a gas where the gas density is low (lower than solids and liquids) and the density of the plasma created by the gas is low as well, limiting its absorption to the medium and far infrared part of the light spectrum. This device is not absorbing in the visible and near-infrared regions and cannot protect in these regions of the spectrum. U.S. Pat. No. 5,017,769 describes the use of a solid insert in the crossover point. This transparent insert is covered with carbon particles on its surface, enhancing the creation of a plasma on the surface at lower light intensities. The plasma density is high, since it starts from solid material. The dense plasma absorbs visible as well as infrared light, and the device is equipped with multiple inserts on a motorized rotating wheel that exposes a new, clean and transparent part after every damaging pulse. The two devices described above, namely U.S. Pat. Nos. 3,433,555 and 5,017,769, are large in their volume, work in free space and require high pulsed powers.
Passive devices were proposed in the past for image display systems. These devices generally contained a mirror that was temporarily or permanently damaged by a high-power laser beam that damaged the mirror by distortion or evaporation. Examples for such devices are described in U.S. Pat. Nos. 6,384,982, 6,356,392, 6,204,974 and 5,886,822. The powers needed here are in the range of pulsed or very energetic CW laser weapons and not in the power ranges for communication or medical devices. The distortion of a mirror by the energy impinging on it is very slow and depends on the movement of the large mass of the mirror as well as the energy creating the move. The process of removing a reflective coating from large areas is also slow, since the mirror is not typically placed in the focus where power is spatially concentrated. Another passive device was proposed in U.S. Pat. No. 621,658B1, where two adjacent materials were used. The first material was heat-absorbing, while the second material was heat-degradable. When these two materials were inserted into a light beam, the first material was heated and transferred its heat to the second material to degrade the transparency or reflectivity of the second material. This process is relatively slow, since heat-transfer times are slow, and in many cases not sufficiently fast to interrupt a light beam before damage occurs to objects along the optical line. In addition, the process of temperature-induced degradation often does not provide enough opacity to efficiently prevent damage from high-power spikes that are a known phenomenon in laser-fiber amplifiers.
Better, faster and more opaque solutions are needed. The present invention provides such a solution.